1. Field of the Invention
The present invention relates to a method and a system of driving a light source for reading or recording in facsimile equipment, optical character readers (OCR) or various types of optical printers which optically read or record a picture image and, more specifically, to an improvement in the control of an application voltage to a light source to prevent the decrease of the luminance of the light source with time.
2. Description of the Related Art
Picture reading apparatus such as facsimile equipment or an OCR is well known wherein a photoelectric converting element such as a contact type image sensor is used to optically read a picture. It is also well known to provide a picture recording apparatus such as an optical printer wherein a light sensitive drum is charged uniformly and exposed with an optical image to form an electrostatic latent image thereon, said electrostatic latent image is developed by toner and said toner is transferred onto a paper for recording of the picture image. These picture reading apparatus and picture recording apparatus require a light source as an essential optical element in addition to a contact type image sensor and a light sensitive drum. For example, the picture reading section of the facsimile equipment is arranged as shown in FIG. 5 and includes a fluorescent indicator tube 1 as a light source. The fluorescent indicator tube 1 is lit up in synchronism with moving of an original paper 2 to illuminate the paper 2. Light reflected by the paper 2 is passed through a lens 3 known as a SELFOX lens and an optical image is focused on photoelectric converting elements 40 of an image sensor 4 and converted to an electric signal according to the density of the original image. This electric signal is amplified and calculated by a group of driver IC's 41 and then stored in a memory section (not shown) or the like, completing the reading operation of the picture. Referring to FIG. 6, the fluorescent indicator tube 1 comprises a two-electrode vacuum tube which has filaments 10, terminals 11 and an anode 12 on which phosphor 13 such as zinc oxide (ZnO) is coated. In operation, thermions emitted from the filaments 10 are accelerated by a voltage applied from the terminals 11 to the anode 12 to excite the phosphor 13, thus lighting up the indicator tube 1.
In case of an optical printer, the optical picture recording section is arranged as shown in FIG. 7 or 8. FIG. 7 uses such a fluorescent indicator tube as mentioned above or a DC type EL panel 5 while FIG. 8 uses a dot array fluorescent light source 6 comprising a plurality of light source dots as a light source of the optical section. More specifically, in the optical printer having such an arrangement as shown in FIG. 7, light emitted from, for example, the DC type EL panel 5 is focused on a light sensitive drum 52 through a shutter array (for example, a PLZT shutter array or liquid crystal shutter array) 50 electro-optically opened according to the picture image and through a condensing lens 51. In the optical printer of such an arrangement as shown in FIG. 8, on the other hand, the respective dot light sources of the fluorescent indicator tube dot array 6 are lit up selectively according to the picture image so that light emitted from the dot light sources forms a picture image on the light sensitive drum 52 through the condensing lens 51. An electrostatic latent image formed on the light sensitive drum 52 by the focused image is developed by toner and the toner image is transferred onto a paper, achieving recording of the picture image. In the prior art picture reading and recording apparatus mentioned above, the light source is controllably driven only by turning ON and OFF voltage applied to the anode in such a manner that in a resulting or recording operation, a positive drive voltage for example is applied to the anode of each light source whereas, when the reading or recording operation is not necessary, the associated light source is put out by ceasing to apply the positive drive voltage (Japanese Application No. 124098/1983).
The prior-art light sources driven by the abovementioned manner have such luminance-to-time characteristics as shown in FIG. 9. A curve a in FIG. 9 indicates a time variation in the luminance of the light source when the anode application voltage is 35 V. It will be seen that the curve a has a very short duration in which a light quantity level Lb required for the reading or recording scan is secured. Even when the application voltage is increased to 50 V for the purpose of getting a higher luminance, the luminance is decreased at a higher rate to a constant level. As appreciated from the foregoing explanation, the prior art method and system of driving the light source have following problems.
1. Since the luminance of the light source is greatly decreased with time, it is impossible to obtain a stable, continuous reading or recording operation.
2. When it is desired to get a high luminance, the decrease in luminance must be taken into consideration and thus the application voltage must be made high, leading to the short life of the light source.
3. It is difficult to stably provide a high luminance, which inevitably lowers the reading or recording speed.